Alkylene carbonates can be prepared through the catalytic reaction of alkylene oxide and carbon dioxide. Alkylene carbonates are useful products and can be used as chemical intermediates or for shipment of alkylene oxide values wherein the alkylene carbonate is transported and subjected to thermal degradation to generate the alkylene oxide. Alkylene carbonates can also be hydrolyzed to form glycols in high selectivity and purity.
Alkylene oxides which have been proposed for the preparation of alkylene carbonates, e.g., see U.S. Pat. No. 2,773,070, issued Dec. 4, 1956, include those represented by the formula ##STR1## wherein W, Y and Z may be the same or different and may be hydrogen, alkyl of 1 to 20 carbon atoms, aryl of 6 to 12 carbon atoms, cycloalkyl containing 5 to 20 carbon atoms, alkenyl of 2 to 20 carbon atoms, and haloalkyl of 2 to 20 carbon atoms, and any of two of W, Y and Z may be interconnected to form with the two carbon atoms shown in the formula, a carbocyclic ring.
The formation of the cyclic carbonate has been by the catalytic reaction of carbon dioxide with alkylene oxide under elevated temperature and pressure in the presence of a catalyst. Numerous homogeneous catalysts have been proposed including ammonium halides (U.S. Pat. No. 2,773,070) and alkali and alkaline earth halides (U.S. Pat. Nos. 2,667,497 and 2,924,608, Japanese Patent Application Kokai No. 23175/63, published Oct. 31, 1963, and U.S.S.R. Author's Certificate No. 170,529, published Apr. 23, 1965 by S. Z. Levin and A. L. Shapiro).
In order to provide an attractive process for the preparation of alkylene carbonates, the process should be capable of achieving high selectivity to alkylene carbonate and should be economically sound. For example, when ethylene oxide is used to form ethylene carbonate which is then hydrolyzed to form ethylene glycol or decomposed to form ethylene oxide, a viable commercial process will be characterized as one which does not incur undue additional costs over the production of ethylene oxide or ethylene glycol directly from the hydrolysis of ethylene oxide. Many factors affect the economics of alkylene oxide manufacture, and one of these factors is the ability to recover and reuse catalyst for the formation of alkylene carbonate.
For purposes of illustration, reference shall be made to continuous processes for the preparation of ethylene carbonate from ethylene oxide using a potassium iodide catalyst. See U.S. Pat. No. 4,314,945, issued Feb. 9, 1982, herein incorporated by reference, for further background information. In these types of processes, ethylene oxide is reacted with carbon dioxide at a temperature of up to about 200.degree. C. under carbon dioxide pressure in the presence of about 0.1 to 3 weight percent potassium iodide. Selectivities of greater than 99 percent with 99.5 percent conversion can be obtained. The catalyst may then be removed from the reaction product by distillation and returned to the carbonate forming reaction zone. During the reaction, other products are formed, albeit in small amounts, and, because of their high boiling temperatures, may be recycled with the catalyst. These other products frequently include polyglycols. Polyglycols that are recycled with the catalyst can lead to an undesirable build up of polyglycols within the reaction zone. Polyglycols are believed to enter into undesirable reactions in the carbonate forming reaction zone that not only affect the efficiency of the process but also can deleteriously affect the quality of the alkylene carbonate. To prevent a build-up of other products recycling with the catalyst, a purge stream may be taken. This, however, can result in the loss of catalyst unless catalyst is recovered from the purge stream. For instance, for the sake of a perspective to the significance of loss of catalyst, in a plant producing 140,000 metric tons of ethylene carbonate per year, a purge stream can result in a removal of about 35 pounds of potassium iodide per hour. At a price of potassium iodide of $10 per pound the value of catalyst lost could be about $2,000,000 per year.
Unfortunately, catalysts such as potassium iodide are not readily recovered from polyglycols contained in the purge since potassium iodide is highly soluble in polyglycols. For example, the solubility of potassium iodide in a synthesized glycol mixture (25.10 weight percent monoethylene glycol, 12.95 weight percent diethylene glycol, 7.16 weight percent triethylene glycol, 9.69 weight percent tetraethylene glycol, 38.72 weight percent ethylene glycol, and 5.06 weight percent hexaethylene glycol) is about 27 weight percent at 25.degree., 130.degree. and 160.degree. C. Indeed, no precipitation of potassium iodide was observed at dry ice-acetone temperature (-78.degree. C.) or at 190.degree. C. This solubility is believed to be due to the chelating effect of the glycols on potassium iodide. Accordingly, an effective system is sought to recover catalyst from such purge streams.